What is Gametogenesis?
Gametogenesis is the process of division of diploid cells to produce new haploid cells.
- In humans, two different types of gametes are present. Male gametes are called sperm and female gametes are called the ovum.
- Gametogenesis occurs when a haploid cell (n) is formed from a diploid cell (2n) through meiosis.
- in male, we call gametogenesis spermatogenesis and it produces spermatozoa.
- In the female, we call it oogenesis. It results in the formation of ova.
What is Spermatogenesis?
Spermatogenesis is the process of formation of haploid male gametes or sperms or spermatozoa from male germ cells or spermatogonia.
- Spermatogenesis is the origin and development of the sperm cells within the male reproductive organs, the testes. Sperm cells are produced within the testes in structures called seminiferous tubules.
- Spermatogenesis can be divided into three phases:
1. Proliferation and differentiation of spermatogonia
- A complex process that transforms round spermatids after meiosis into a complex structure called the spermatozoon.
In the Beginning
- Males start producing sperm when they reach puberty, which is usually from 10-16 years old. Biological males continually produce sperm in large quantities (~200 million a day). This maximises the likelihood of sperm reaching the egg following ejaculation.
- Sperm production occurs in the testes of the male, specifically in the seminiferous tubules. In the testicles, a blood-testis barrier forms to keep the tubules separate from the systemic circulation.
Protecting the Sperm
- Sertoli cells form the blood-testis barrier. This is important in preventing substances found in blood from affecting the developing sperm. These products might include hormones or waste products.
- It is also important as it prevents the immune system of the male from recognising the sperm as foreign – the sperm are genetically different from the male and will express different surface antigens.
Forming Functional Sperm
- Spermatogonia are the initial pool of diploid cells that divide by mitosis to give two identical cells. One of these cells will be used to replenish the pool of spermatogonia – these cells are A1 spermatogonia. This replenishment of spermatogonia means that males are fertile throughout their adult life. The other cell – type B spermatogonium – will eventually form mature sperm.
- Type B spermatogonia replicate by mitosis several times to form identical diploid cells linked by cytoplasm bridges, these cells are now known as primary spermatocytes. Primary spermatocytes then undergo meiosis.
(i) Meiosis I produces two haploid cells, known as secondary spermatocytes.
(ii) Meiosis II produces four haploid cells, known as spermatids.
- The cytoplasmic bridges break down and the spermatids are released into the lumen of the seminiferous tubule – a process called spermiation. The spermatids undergo spermiogenesis (remodelling and differentiation into mature spermatozoa) as they travel along the seminiferous tubules until they reach the epididymis.
- From the seminiferous tubule, cells will travel to the rete testis. This acts to “concentrate” the sperm by removing excess fluid. Then, cells move to the epididymis where the sperm is stored and undergoes the final stages of maturation.
- Spermatogenesis takes approximately 70 days, therefore in order for sperm production to be continuous and not intermittent, multiple spermatogenic processes are occurring simultaneously within the same seminiferous tubule, with new groups of spermatogonia arising every 16 days (spermatogenic cycle). Each of these populations of spermatogenic cells will be at different stages of spermatogenesis.
- Note that once sperm leave the male body and enter the female reproductive tract, the conditions there cause the sperm to undergo capacitation, which is the removal of cholesterol and glycoproteins from the head of the sperm cell to allow it to bind to the zona pellucida of the egg cell.
Diagram showing process of Spermatogenesis
Factors Affecting Spermatogenesis
- This process seems to be very sensitive and can easily be affected by minute changes in the hormone level.
- For example- testosterone is developed through the hypothalamus, Leydig cells, and pituitary gland.
- This process is very sensitive to changes in temperature, deficiency in the diet, alcoholism, exposure to drugs and the presence of disease can affect the rate of sperm formation adversely.
Try yourself:What is the process of conversion of spermatids to sperms called?
- Spermatids are produced as a result of meiosis of spermatogonia and thus are haploid. These spermatogonia mature and differentiate further to produce sperms by the process of spermiogenesis
Let's discuss other terms as well:
Spermiation is the process by which mature spermatids are released from Sertoli cells into the seminiferous tubule lumen prior to their passage to the epididymis.
Spermatogenesis is the origin and development of the sperm cells within the male reproductive organs, the testes. Sperm cells are produced within the testes in structures called seminiferous tubules.
Meiosis, also called reduction division, division of a germ cell involving two fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell.
What is Oogenesis?
Oogenesis is the process of the formation of female gametes.
- Oogenesis differs from spermatogenesis in that it begins in the foetus prior to birth. Primordial germ cells (which originate in the yolk sac of the embryo) move to colonise the cortex of the primordial gonad. Replication by mitosis peaks at approximately 7 million by mid-gestation (~20 weeks).
- Cell death occurs after this peak to leave 2 million cells. Meiosis I begins before birth and forms primary oocytes. There is therefore a finite supply of ova.
- Primary oocytes are arranged in the gonads as clusters. They have flattened epithelial cells surrounding them, and this is called the primary follicle.
- During childhood, further atresia (cell death) occurs, leaving ~40,000 eggs at puberty.
- Once puberty begins, a number of primary oocytes (15-20) begin to mature each month, although only one of these reaches full maturation to become an oocyte.
- The primary oocytes undergo 3 stages:
Structure of Ovary
Phases of Oogenesis
There are three phases to oogenesis, namely:
- Multiplication Phase
- Growth Phase
- Maturation Phase
Let us try to understand these phases in a precise manner:
- Multiplication Phase: During foetal development, it should be noticed that certain cells present in the germinal epithelium of the female ovary are bigger than others. Hence, these cells split by mitosis, creating a couple of million oogonia or mother egg cells in each ovary present in the foetus. There are no more oogonia that are formed or augmented after birth.
- Growth Phase: This particular procedure of the primary oocyte tends to be very long. In this, the oogonium nurtures into bigger primary oocytes. After this, each primary oocyte gets surrounded by a granulosa cells layer to create a primary follicle. Later, a large number of follicles get debased during the duration from birth to puberty. Therefore, at puberty around 60,000 to 80,000 primary follicles can be found in each ovary.
- Maturation Phase: Similar to a primary spermatocyte, every primary oocyte experiences two maturation divisions. However, the outcomes of maturation divisions under oogenesis are quite different to those which occur in spermatogenesis. Considering the first meiotic division, the primary oocyte segregates into two uneven haploid daughter cells. These are known as the large secondary oocyte and a small polocyte.
Later, considering the second maturation separation, the initial polar body might split to create two, second polar bodies. Here, the secondary oocyte once again divides to form unfit daughter cells.
Process of Oogenesis
- Now, the follicle has grown in size and is mature – it is called a Graafian follicle.
- An LH surge occurs and increases collagenase activity. This is an enzyme that disrupts collagen. Therefore, there is a weakening of the follicular wall.
- This, combined with muscular contractions of the ovarian wall, results in the ovum being released from the ovary.
- The ovum is then taken up into the fallopian tube via the fimbriae (finger-like projections of the fallopian tube).
Fertilisation: Final Stage of Female Gametogenesis
The secondary oocyte will only complete meiosis II following fertilisation. Here, it gives off a third polar body. Following meiosis II, a fertilised egg results. If fertilisation doesn’t occur, the oocyte degenerates 24 hours after ovulation, remaining arrested in meiosis II. If fertilisation does occur, peristaltic movements of the fallopian tube move the egg to the uterus where it can implant into the posterior uterine wall.
Difference Between Spermatogenesis and Oogenesis
Try yourself:What are the cells that secondary oocyte divides into called?
Secondary oocyte is formed as a result of the asymmetric division of primary oocyte, which also produces a tiny first polar body. Secondary oocyte further undergoes another asymmetric division producing a larger ovum and a smaller second polar body.